Crystal oscillator

ABSTRACT

An oscillator circuit may be operated in a high power mode or a reduced power mode. The high power mode provides fast start-up of the oscillator circuit.

RELATED APPLICATION

This application is a Continuation of U.S. Nonprovisional ApplicationSer. No. 11/013,059, by Stevenson et al., filed Dec. 15, 2004, which isincorporated herein by reference in its entirety for all purposes.

FIELD

The present invention relates to oscillator circuits, and morespecifically to crystal oscillator circuits.

BACKGROUND

Oscillator circuits are useful to create oscillating signals. Ingeneral, low power crystal oscillator circuits may take a substantialperiod of time to begin oscillating with an amplitude sufficiently largeto be useful. Start-up time can be reduced, but it may be at the expenseof increased power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of an integrated circuit and a crystal;

FIGS. 2 and 3 show crystal oscillator circuits in accordance withvarious embodiments of the present invention;

FIG. 4 shows a flowchart in accordance with various embodiments of thepresent invention; and

FIG. 5 shows an electronic system in accordance with various embodimentsof the present invention.

DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

FIG. 1 shows a diagram of an integrated circuit and a crystal. Crystal110 may be any type of material having a resonant frequency. Forexample, crystal 110 may be a crystal resonator having one or moreoscillation modes with different mechanical oscillation frequencies. Anytype of resonator may be utilized in place of crystal 110 withoutdeparting from the scope of the present invention.

Integrated circuit 100 includes oscillator circuit 120 and start-upcontrol circuit 130. Integrated circuit 100 also includes electricalcontacts 102 and 104 at which oscillator circuit 120 is coupled tocrystal 110. Oscillator circuit 120 senses the oscillations of crystal110 and amplifies them to provide an oscillation signal of sufficientamplitude to be useful to integrated circuit 100. For example,oscillator circuit 120 may provide a clock signal (not shown) having anamplitude suitable to drive logic circuits (not shown) within integratedcircuit 100.

Integrated circuit 100 may be any type of integrated circuit thatincludes an oscillator circuit. For example, in some embodiments,integrated circuit 100 may be a microprocessor, a digital signalprocessor, an embedded processor, a microcontroller, a system on a chip,or the like. In other embodiments, integrated circuit 100 may be limitedto oscillator circuitry and associated peripheral circuitry. Further,integrated circuit 100 may include many circuits and functional blocksin addition to those shown in FIG. 1 without departing from the scope ofthe present invention. For example, integrated circuit 100 may includean arithmetic logic unit (ALU), a volatile or nonvolatile memory, agraphics processor, or the like.

Start-up control circuit 130 provides various control signals to, andreceives feedback from, oscillator circuit 120. For example, start-upcontrol circuit 130 provides one or more loop filter control signals onnode 132 and also provides one or more bias current control signals onnode 134. The signals provided to oscillator circuit 120 from start-upcontrol circuit 130 may be utilized to control an operating mode ofoscillator circuit 120. For example, by controlling a bias current,start-up control circuit 130 may cause oscillator circuit 120 to operatein a low power mode or a high power mode. Further, loop filter controlsignals on node 132 may be utilized to vary loop filter characteristicswithin oscillator circuit 120.

Oscillator circuit 120 may be started as a result of a “start-up event.”Examples of start-up events include the application of power tointegrated circuit 100 (e.g., turning on a power switch), awaking from asleep mode, or the like. When starting, oscillator circuit 120 may beput into a start-up mode by start-up control circuit 130. Oscillatorcircuit 120 may be considered as “started” when an operating conditionis met. For example, operating conditions may include a time ofoperation or the oscillation amplitude reaching a predetermined level.

In some embodiments, oscillator circuit 120 may be put in a high powerstart-up mode as a result of start-up event to allow the oscillator tostart quickly. Further, oscillator circuit 120 may be put in a reducedpower operating mode when an operating condition is met. The combinationof modes allows the oscillator circuit to start quickly and to operateover a long period of time with reduced power consumption.

In some embodiments, start-up control circuit 130 receives feedback fromoscillator circuit 120 on node 122, and modifies loop filter controlsignals and bias current control signals in response. For example, insome embodiments, the feedback on node 122 may provide an indication ofthe oscillation amplitude of oscillator circuit 120, and start-upcontrol circuit 130 may include circuitry to determine when thatamplitude is sufficiently large to change the mode of operation.Further, in some embodiments, start-up control circuit 130 may modifyloop filter control signals and bias current control signals afteroscillator circuit 120 has been operating for a predetermined amount oftime.

Start-up control circuit 130 may include any type of circuitry capableof receiving feedback from, and providing signals to, oscillator circuit120. For example, start-up control circuit 130 may include digitalcircuits such as counters, latches, registers, state machines, or thelike. Also for example, start-up control circuit 130 may include analogcircuits such as amplitude detectors, envelope detectors, comparators,operational amplifiers, or the like.

FIG. 2 shows a crystal oscillator circuit in accordance with variousembodiments of the present invention. Crystal oscillator circuit 200includes transistors 202, 204, 206, 208, 210, 212, and 214; capacitors224, 226, 228, 230, 232, and 234; and resistors 240, 242, 244, and 246.Also shown in FIG. 2 are amplitude detector 260, control circuit 270,switch 280, and crystal 250.

The transistors in FIG. 2 and later figures are shown as isolated gatetransistors, and specifically as metal oxide semiconductor field effecttransistors (MOSFETs). For example, transistors 202, 204, and 206 areshown as P-type MOSFETs, and the remaining transistors are shown asN-type MOSFETs. Other types of switching or amplifying elements may beutilized for the various transistors of oscillator circuit 200 withoutdeparting from the scope of the present invention. For example, thetransistors of circuit 200 may be junction field effect transistors(JFETs), bipolar junction transistors (BJTs), or any device capable ofperforming as described herein.

Transistors 204, 208, 202, and 210, and resistors 244 and 246 combine toform a bias current generator that generates a selectable bias currentshown as I_(BIAS) in FIG. 2. Associated with these transistors are tworesistor-capacitor (RC) networks. Resistor 240 and capacitor 232 combineto form a first RC network, and transistor 214 and capacitor 234 combineto form a second RC network where the resistance of the RC network isprovided by the drain-to-source resistance of transistor 214. Theresistance of transistor 214 may be controlled by the filter controlsignal on node 272, thereby modifying the time constant of the RCnetwork. For example, when a high voltage is presented on node 272 bycontrol circuit 270, the drain-to-source resistance of transistor 214 isrelatively small, and capacitor 234 may charge fairly quickly. Also forexample, when a lower voltage is presented on node 272, thedrain-to-source resistance of transistor 214 is increased and the timeconstant of the RC network is correspondingly increased.

The current I_(BIAS) is mirrored through transistor 204 to create I₁,and is also mirrored through transistor 206 to create I₂. Current I₁ isprovided to transistor 208, and node 207 receives a direct current (DC)voltage as a result of resistor 240 being coupled drain-to-gate ontransistor 208. Current I₂ is provided to transistor 212, which iscoupled as a gain element to amplify oscillations of crystal 250. Thegain of transistor 212 increases with increased I₂, as does the powerdissipation of circuit 200.

Crystal 250 is coupled between the gate node and drain node oftransistor 212, and as the oscillations grow in amplitude, analternating current (AC) signal appears on the gate of transistor 212.Capacitors 228 and 230 form a capacitive voltage divider that dividesthe AC voltage present on node 211 to provide an AC voltage component onnode 207. The AC voltage component on node 207 is proportional to theamplitude of the crystal's oscillation. As the oscillation amplitudegrows, the DC voltage on the drain of transistor 208 drops in proportionto the increase in oscillation amplitude.

As the voltage on the drain of transistor 208 drops as a result of theoscillation amplitude increasing, the voltage on the gate of transistor210 will also drop, thereby reducing I_(BIAS). Accordingly, transistor214 and capacitor 234 form a loop filter in an amplitude regulationfeedback loop. As the oscillation amplitude increases, I_(BIAS)decreases, which reduces 12 and reduces the oscillation amplitude.

During operation, a long time constant for the loop filter (transistor214 and capacitor 234) can prevent the feedback loop from becomingunstable. The time constant may be held high by controlling transistor214 to provide a relatively high resistance, but when transistor 214provides a high resistance, it may take a long time for node 209 toinitially charge up when the oscillator circuit is started.

In various embodiments of the present invention, control circuit 270drives a high voltage on node 272 to reduce the time constant of theloop filter during start-up of the oscillator circuit. By reducing thetime constant of the loop filter during start-up, node 209 may chargefaster and I_(BIAS) may be established faster, thereby allowing theoscillator circuit to start faster. In these embodiments, after theoscillator has started, control circuit 270 may drive a lower voltage onnode 272 to increase the time constant of the loop filter to prevent theamplitude regulation loop from becoming unstable.

As shown in FIG. 2, transistor 214 provides a variable resistance. Insome embodiments, a variable resistor is implemented in a differentmanner. For example, in some embodiments, a variable resistance isprovided by multiple resistors coupled in series or parallel withswitches to include or remove the resistors from the loop filtercircuit. The various embodiments of the present invention are notlimited by the manner in which the variable resistance is implemented.

As shown in FIG. 2, the time constant of the loop filter is modified bychanging the resistance of transistor 214. In some embodiments, the timeconstant of the loop filter is modified by changing the capacitance ofcapacitor 234. For example, capacitor 234 may include multiplecapacitors that may be individually included or excluded from thecircuit.

In some embodiments, amplitude detector 260 detects the oscillationamplitude present on node 211. For example, amplitude detector 260 mayinclude a diode detector or any other type of circuit that is capable ofdetecting the amplitude of the oscillations on node 211. Amplitudedetector 260 may also include a comparator to compare the amplitude to athreshold. For example, the oscillator circuit may be considered in astart-up mode when the amplitude is below a particular threshold, andmay be considered in a normal operating mode when the amplitude is abovethe threshold. By comparing the detected amplitude to a threshold,amplitude detector 260 may determine when the oscillator circuit is tobe transitioned from start-up mode to normal operating mode.

Amplitude detector 260 and control circuit 270 are part of a controlmechanism to operate oscillator circuit 200 in either a start-up mode oran operating mode. Amplitude detector 260 receives feedback from theoscillator on node 211, and control circuit 270 receives informationfrom amplitude detector 260. In some embodiments, the informationreceived by control circuit 270 may include the detected amplitude. Alsoin some embodiments, the information received may include one or moredigital signals representing the amplitude crossing one or morethresholds. In response to the information received from amplitudedetector 260, control circuit 270 may manipulate the filter controlsignal on node 272 as described above, and may manipulate the biascontrol signal on node 274 as described below.

In addition to modifying the loop filter during the start-up ofoscillator circuit 200, control circuit 270 may modify the biasresistance coupled to the source of transistor 210 to increase I_(BIAS)during the start-up of oscillator circuit 200. For example, duringstart-up, control circuit 270 may cause switch 280 to close, therebyreducing the bias resistance, and increasing the bias current I_(BIAS).When the bias current I_(BIAS) is increased, 12 is also increased,allowing the oscillator to start quickly. When the oscillator circuithas started, control circuit 270 may open switch 280, thereby decreasingthe bias current I_(BIAS). When the bias current is decreased, I₂ isalso decreased, and the oscillator consumes less power.

Resistors 244 and 246 and switch 280 form a variable resistor that canaffect the bias current I_(BIAS). Oscillator circuit 200 is shown havingtwo possible settings for the variable resistor coupled to the source oftransistor 210. In some embodiments, many more possible settings existfor bias resistance. For example, multiple resistors and multipleswitches may be arranged to provide different amounts of biasresistance. Multiple resistors of different values may be combined inseries or parallel combinations to create changes in bias resistance ofany type. For example, the bias resistance can be changed in linearsteps, exponential steps, or in any other fashion. In some embodiments,the bias resistance may be changed in increments based on theoscillation amplitude crossing various thresholds.

FIG. 3 shows a crystal oscillator circuit in accordance with variousembodiments of the present invention. Oscillator circuit 300 includesmany of the transistors and capacitors shown in FIG. 2. Also shown inFIG. 3 are counter 360 and control circuit 370. In various embodimentsof the present invention, counter 360 counts the number of cycles of theoscillating signal on node 211, and provides a count value to controlcircuit 370 on signal nodes 362. In response to counter values providedon signal nodes 362, control circuit 370 manipulates one or more of theloop filter control signals on nodes 372 and 373 and manipulates thebias resistor control signals on nodes 374.

As shown in FIG. 3, oscillator circuit 300 includes variable capacitor334 as part of the loop filter circuit. In some embodiments, variablecapacitor 334 includes multiple capacitors and switches that areconfigured to either switch capacitors into the circuit, or switchcapacitors out of the circuit. For example, variable capacitor 334 mayinclude capacitors fabricated using gates of transistors,metal-insulator-metal (MIM) capacitors, or any other type of capacitorsupported by the manufacturing processes employed. Also for example,switches may be implemented using transistors with a relatively low “on”resistance, and a relatively high “off” resistance.

The loop filter in FIG. 3 is shown having a variable resistive element(transistor 214) and a variable capacitor. In some embodiments, avariable resistor is employed with a fixed capacitor. In otherembodiments, a variable capacitor is employed with a fixed resistor. Anycombination of variable circuit elements may be utilized withoutdeparting from the scope of the present invention.

Oscillator circuit 300 is put into a start-up mode upon a startingevent, and is put into an operating mode when an operating condition ismet. In some embodiments, the operating condition may be represented bya time period. For example, counter 360 counts a time period that theoscillator is running by counting cycles of the oscillating signal, andcontrol circuit 370 may detect when the time period crosses one or morepredetermined thresholds. Control circuit 370 may then change the modeof the oscillating circuit.

In some embodiments, control circuit 370 detects when oscillator circuit300 has been running for a predetermined period of time, and changes theloop filter control to provide a larger time constant as described abovewith reference to FIG. 2. Also in some embodiments, control circuit 370may change bias control signals on nodes 374 to change the value ofI_(BIAS).

The variable resistor shown coupled to the source of transistor 210 inFIG. 3 includes four resistor segments instead of the two resistorsegments shown in FIG. 2. Each resistor segment may be included orremoved from the circuit through the action of switches 380, 382, and384. The arrow shown at the end of signal nodes 374 is meant to indicatethat signals on signal nodes 374 control switches 380, 382, and 384.Although four resistor segments and three switches are shown in FIG. 3,this is not a limitation of the present invention. For example, anynumber of resistor segments and any number of switches may be includedin oscillator circuit 300.

Control circuit 370 may increase the bias resistance in steps by closingswitches 380, 382, and 384 in a coordinated sequence. For example, whencounter 360 reaches a first count value, control circuit 370 may closeswitch 384. When counter 360 reaches a second count value, controlcircuit 370 may close switch 382, and when counter 360 reaches a thirdcount value, control circuit 370 may close switch 380.

In addition to modifying the bias resistance in increments, controlcircuit 370 may control the time constant of the loop filter inincrements. For example, control circuit 370 may apply a high voltage onnode 372 upon a start-up condition, and then may gradually reduce thevoltage on node 372 as the count value increases, so as to increase thetime constant of the loop filter gradually as the time of operationincreases. In addition, control circuit 370 may change the time constantof the loop filter by incrementally changing a capacitance value ofvariable capacitor 334. In still further embodiments, control circuit370 may modify both the resistance of transistor 214 and the capacitanceof variable capacitor 334.

Oscillator circuits, control circuits, transistors, and otherembodiments of the present invention can be implemented in many ways. Insome embodiments, they are implemented in integrated circuits. In someembodiments, design descriptions of the various embodiments of thepresent invention are included in libraries that enable designers toinclude them in custom or semi-custom designs. For example, any of thedisclosed embodiments can be implemented in a synthesizable hardwaredesign language, such as VHDL or Verilog, and distributed to designersfor inclusion in standard cell designs, gate arrays, custom devices, orthe like. Likewise, any embodiment of the present invention can also berepresented as a hard macro targeted to a specific manufacturingprocess. For example, oscillator circuit 200 (FIG. 2) may be representedas polygons assigned to layers of an integrated circuit.

FIG. 4 shows a flowchart in accordance with various embodiments of thepresent invention. In some embodiments, method 400 may be used in, orfor, a crystal oscillator circuit. In some embodiments, method 400, orportions thereof, is performed by a start-up control circuit,embodiments of which are shown in the various figures. In otherembodiments, method 400 is performed by an amplitude detector andcontrol circuit, a counter and control circuit, or an electronic system.Method 400 is not limited by the particular type of apparatus performingthe method. The various actions in method 400 may be performed in theorder presented, or may be performed in a different order. Further, insome embodiments, some actions listed in FIG. 4 are omitted from method400.

Method 400 is shown beginning at block 410 in which a crystal oscillatorcircuit is started in a high power mode to provide a fast start. In someembodiments, this may correspond to reducing the resistance of a biasresistor in a bias current generator to increase the current provided toa gain device coupled to a crystal. For example, referring back to FIG.2, control circuit 270 may reduce the value of the resistance coupled tothe source of transistor 210 to increase I_(BIAS). Further, in someembodiments, the acts of block 410 may also correspond to reducing thetime constant of a loop filter as described above with reference toFIGS. 2 and 3.

At 420, method 400 determines whether the crystal oscillator circuit hasstarted. In some embodiments, this corresponds to determining if anoperating condition is met. For example, the operating condition mayinclude an amount of time that the oscillator circuit has beenoperating, or may include an oscillation amplitude. At 430, the crystaloscillator circuit is run in a low power mode to save power. In someembodiments, this corresponds to increasing the value of a bias resistorto reduce a bias current and to reduce the current provided to a gaindevice coupled to a crystal.

At 440, a loop filter time constant is modified in the crystaloscillator circuit. In some embodiments, this corresponds to changing agate voltage on a transistor that provides a resistance in an RCnetwork. The time constant of the loop filter may be changed in onelarge step, or may be changed in smaller increments over time.

FIG. 5 shows a system diagram in accordance with various embodiments ofthe present invention. FIG. 5 shows system 500 including system-on-chip(SOC) 510, receiver 530, and antennas 540. SOC 510 includes oscillatorcircuit 520 coupled to crystal 522 at electrical contacts 514 and 516.Oscillator circuit 520 may include any of the oscillator circuitembodiments represented by the previous figures. In some embodiments,SOC 510 is an integrated circuit that includes many components; however,as used herein, the term “system-on-chip” and the acronym “SOC” do notimply any particular level of integration. For example, in someembodiments, an SOC may include a processor and an oscillator circuit,or a peripheral device and an oscillator circuit.

In systems represented by FIG. 5, SOC 510 is coupled to receiver 530 byconductor 512. Receiver 530 receives communications signals fromantennas 540 and also communicates with SOC 510 on conductor 512. Insome embodiments, receiver 530 provides communications data to SOC 510.Also in some embodiments, SOC 510 provides control information toreceiver 530 on conductor 512.

Example systems represented by FIG. 5 include cellular phones, personaldigital assistants, wireless local area network interfaces, and thelike. Many other systems uses for SOC 510 exist. For example, SOC 510may be used in a desktop computer, a network bridge or router, or anyother system without a receiver.

Receiver 530 includes amplifier 532 and demodulator (demod) 534. Inoperation, amplifier 532 receives communications signals from antennas540, and provides amplified signals to demod 534 for demodulation. Forease of illustration, frequency conversion and other signal processingis not shown. Frequency conversion can be performed before or afteramplifier 532 without departing from the scope of the present invention.In some embodiments, receiver 530 may be a heterodyne receiver, and inother embodiments, receiver 530 may be a direct conversion receiver. Insome embodiments, receiver 530 may include multiple receivers. Forexample, in embodiments with multiple antennas 540, each antenna may becoupled to a corresponding receiver.

Receiver 530 may be adapted to receive and demodulate signals of variousformats and at various frequencies. For example, receiver 530 may beadapted to receive time domain multiple access (TDMA) signals, codedomain multiple access (CDMA) signals, global system for mobilecommunications (GSM) signals, orthogonal frequency division multiplexing(OFDM) signals, multiple-input-multiple-output (MIMO) signals,spatial-division multiple access (SDMA) signals, or any other type ofcommunications signals. The various embodiments of the present inventionare not limited in this regard.

Antennas 540 may include one or more antennas. For example, antennas 540may include a single directional antenna or an omni-directional antenna.As used herein, the term omni-directional antenna refers to any antennahaving a substantially uniform pattern in at least one plane. Forexample, in some embodiments, antennas 540 may include a singleomni-directional antenna such as a dipole antenna, or a quarter waveantenna. Also for example, in some embodiments, antennas 540 may includea single directional antenna such as a parabolic dish antenna or a Yagiantenna. In still further embodiments, antennas 540 include multiplephysical antennas. For example, in some embodiments, multiple antennasare utilized for multiple-input-multiple-output (MIMO) processing orspatial-division multiple access (SDMA) processing.

Although SOC 510 and receiver 530 are shown separate in FIG. 5, in someembodiments, the circuitry of SOC 510 and receiver 530 are combined in asingle integrated circuit. Furthermore, receiver 530 can be any type ofintegrated circuit capable of processing communications signals. Forexample, receiver 530 can be an analog integrated circuit, a digitalsignal processor, a mixed-mode integrated circuit, or the like.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within the scopeof the invention and the appended claims.

1. An oscillator circuit comprising: means for generating a selectablebias current; a loop filter with a variable time constant to influencean oscillation amplitude; and means to influence operation of the loopfilter and to select a bias current.